1. Field of the Invention
The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and separators alternately. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. Further, the present invention relates to a fuel cell stack formed by stacking a plurality of fuel cells.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
Some of the fuel cell stacks of this type formed by stacking a plurality of fuel cells are known from, for example, Japanese Laid-Open Patent Publication No. 2002-203579, which discloses a solid oxide fuel cell. As shown FIG. 21, the solid oxide fuel cell is formed by stacking power generation cells 1 and separators 2 alternately. Each of the power generation cells 1 includes a fuel electrode layer 1b, an air electrode layer 1c, and a solid electrolyte layer 1a interposed between the fuel electrode layer 1b and the air electrode layer 1c. A porous conductive fuel electrode current collector 3 is provided on one surface of the power generation cell 1, and a porous conductive air electrode current collector 4 is provided on the other surface of the power generation cell 1. The fuel electrode current collector 3, the power generation cell 1, and the air electrode current collector 4 are sandwiched between a pair of separators 2.
The separator 2 has a fuel gas supply passage 5 and an air supply passage 6. The fuel gas supply passage 5 is connected to a fuel gas hole 5a formed at a substantially central region on one surface of the separator 2. The air supply passage 6 is connected to an air hole 6a formed at a substantially central region on the other surface of the separator 2. The fuel gas hole 5a faces the fuel electrode current collector 3. The air hole 6a faces the air electrode current collector 4.
The fuel gas such as H2 or CO flows through the fuel gas supply passage 5, and is discharged from the substantially central region of the separator 2 toward the center of the fuel electrode current collector 3. The fuel gas flows through holes formed in the fuel electrode current collector 3 toward the substantially central region of the fuel electrode layer 1b. Then, the fuel gas flows along unillustrated slits to move radially outwardly toward the outer region of the fuel electrode layer 1b. 
Likewise, the air is supplied from the substantially central region of the separator 2 toward the center of the air electrode current collector 4 through the air supply passage 6. The air flows through holes formed in the air electrode current collector 4 toward the substantially central region of the air electrode layer 1c. Then, the air flows along unillustrated slits to move radially outwardly toward the outer region of the air electrode layer 1c. In this manner, in each of the power generation cells 1, the fuel gas is supplied to the surface of the fuel electrode layer 1b, and the air is supplied to the surface of the air electrode layer 1c to carry out power generation.
When a large number of power generation cells 1 and separators 2 are stacked together as described above, it is necessary to apply a uniform load (pressure) to each of the power generation cells 1. It is desirable to achieve the uniform surface pressure, high performance, and long service life. For these purposes, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 10-241707 is known.
As shown in FIG. 22, according to the disclosure of Japanese Laid-Open Patent Publication No. 10-241707, a power generation cell 7 is sandwiched between a pair of separators 8. The power generation cell 7 includes an electrode plate 7a and electrolyte plates 7b, 7c provided on both surfaces of the electrode plate 7a. A pair of current collector plates 7d are stacked on the outside of the electrode plates 7b, 7c. The separator 8 includes a partition plate 8a, a current collector corrugated plate 8b, a seal frame 8c, and a support frame 8d. When a tightening pressure is applied to the surface of the fuel cell stack during operation of the fuel cell stack, the support frame 8d and the current collector corrugated plate 8b are deformed elastically to substantially the same extent.
However, according to the disclosure of Japanese Laid-Open Patent Publication No. 10-241707, when a tightening load is applied to the seal member to achieve the desired sealing performance, an excessive load may be applied to the electrolyte electrode assembly undesirably, and the electrolyte electrode assembly may be damaged. Further, since the separator 8 includes the partition plate 8a, the current collector corrugated plate 8b, the seal frame 8c, and the support frame 8d, the structure of the separator 8 is complicated, the separator 8 is expensive, and the thickness of the separator 8 is considerably large. Therefore, the power generation capacity per unit volume of the fuel cell stack is low. The number of processes required for producing the fuel cell stack is increased, and the production cost of the fuel cell stack is high.